1. Field of the Invention
The present invention relates to an electric power source, an operational method of the electric power source, an inverter and an operation method of the inverter that has the ability to regulate an output current by adjusting equivalent output impedance by only control parameters.
2. Description of Related Art
A plurality of storage batteries connected in a series are provided in an uninterruptible power supply system for communication, however, the terminal voltage of these storage batteries are often in a sufficiently low state after an interruption of power or immediately after diagnostics. When turning on the power switch in this state, over-current, i.e. a large inrush current, flows resulting in a hazardous situation that can break the switch element; therefore, an inrush current prevention circuit is equipped to the electric power source to prevent against inrush current, or a switch element is placed in the inverter to initiate a soft start, in other words, to execute a soft start control that keeps the pulse width small at first and gradually increases the pulse width. However, when an inrush current prevention circuit is equipped, problems such as the generation of power loss, heat, a drop in reliability, and cost accompany this. A further problem is the requirement for soft start control circuitry when using a soft start control.
Moreover, when a large amount of output capacity is required, a redundancy operation can be performed from the oldest by connecting a plurality of inverters in parallel or rectifying power supply. Further, in order to improve the reliability of a power source system employing the inverters, especially, an uninterruptible power supply system, it is common to connect a plurality of inverters in parallel in a construction having a sufficient level of redundancy to load capacity. Further, when operated redundantly by connecting a plurality of these inverters in parallel, a power switch is equipped in each inverter (see for example, Japanese Patent Application First Publication No. H09-331681). With such type of uninterruptible power supply system, when connecting or disconnecting a certain inverter while in parallel operation, the power switch is always turned ON or OFF; and the power switch is necessarily turned ON or OFF even when connecting parallel connected inverters to a commercial AC electrical power system or when opening.
On the other hand, there are various proposals that relate to constantly-sampled error-tracking-mode inverter technology that the embodiment of the present invention can apply (see for example Japanese Patent Application First Publication No. H07-7950, Japanese Patent Application First Publication No. 2000-125575, and Japanese Patent Application First Publication No. 2000-341956). Brief explanation of the constantly-sampled error-tracking-mode power source provides for a first electric current detection means for detecting the output current of the inverter consisting of an inverter and an output filter circuit connected to the output side thereof, a second electric current detection means for detecting the AC current that flows between the inverters and the output filter circuitry, and an AC current detection means for detecting the voltage of the output filter circuitry. Further, the electric current target function signal J(t) is formed by adding the signal that multiplies the output current feed forward gain β, which is the control parameter, by the electric current detection signal from the first electric current detection means, to the signal obtained by multiplying the output voltage feedback gain α by the voltage signal showing the difference between a reference voltage (a voltage requested, determined or desired beforehand) and the voltage detection signal from the AC voltage detector. Next, the error signal Δt that shows the difference between the electric current detection signal from the second electric current detector and the electric current target function signal J(t) determines whether or not Δt is within the target tracking range at each of the constantly-sampled cycles. A high frequency PWM signal is generated that controls the instantaneous value of the electric current by sampling the error signal Δt, then a switching mode for switching elements of the inverter is selected according to the error signal Δt.
When the error signal Δt is negative, because the output current of the inverter is smaller than the electric current target function signal J(t), the switching mode is selected that increases the output current of the inverter; and when the error signal Δt is positive, because the output current of the inverter is larger than the electric current target function signal J(t), the switching mode is selected that decreases the output current of the inverter thereby controlling the electric current detection signal of the first electric current detection means to be within the prescribed range.
Further, when operating in parallel by connecting a plurality of rectifiers in parallel, the flow of current, i.e. cross-current, is prevented to flow into the other rectifiers from a certain rectifier by connecting a diode, for example, for preventing back-current into the output of each rectifier and connecting in parallel in the subsequent stage thereof. There are other various constructions for preventing cross-current, and since the prevention of cross-current in a rectifier is a simple matter, parallel operation of rectifiers is widely used.
However, in case of undertaking parallel operation of inverters, limiting the cross-current to a small value while controlling the pulse width of each of the inverters to regulate the AC output electrical power is difficult. Voltage differences and current differences occur between inverters when the output voltage approaches the reference value by controlling the pulse width of the switching element of the inverters which makes this technically difficult. According to circumstances, the polarity can be opposite depending on the case; therefore, there are various ideas already proposed for technology to resolve these problematic points.
For example, stability of parallel operation for master and slave inverters has been devised (see for example, Japanese Patent Application First Publication No. H08-205543) by, prior to initializing parallel operation of the master and slave inverters, a phase correction signal is delivered that matches the phase of the zero cross detection signal from the control circuit of the slave inverter to the phase of the zero cross signal from the zero cross detection circuit in the control circuit of the master inverter, and the phase of the standard pulse consisting of the basic control signal of the control circuit is corrected by the phase correction signal. Other inventions for suppressing cross-current are also proposed (see for example, Japanese Patent Application First Publication No. H08-223807, Japanese Patent Application First Publication No. H09-140148, Japanese Patent Application First Publication No. 2001-177997, Japanese Patent Application First Publication No. 2002-262577, and Japanese Patent Application First Publication No. 2004-236496).
In addition, suppressing cross-current flow for parallel operation of a plurality of inverters is also proposed by performing an instantaneous value control to make the internal impedance of each inverter to be equally zero to provide stable control over transitory deviation of the phase and amplitude of the output voltage by inputting each load current corresponding value and output current corresponding value for each inverter into the cross-current detection circuit respectively, and outputting the cross-current corresponding value that corresponds to the cross-current flowing between the inverters, and outputting the valid power deviation and the invalid power deviation from the output voltage corresponding value of the inverters and the cross-current electric current corresponding value, and outputting the voltage control value according to such valid power deviation, and outputting the phase control value from the invalid power deviation (see for example Japanese Patent Application First Publication No. H08-223808).
However, conventional inverters that provide inrush current prevention circuitry or perform inrush current prevention by executing a soft start control have the problems described above when driven by storage batteries or a condenser load.
The electric power source of Japanese Patent Application First Publication No. H09-331681, when closing, separating, or opening for a certain inverter, cross-current increases due to the unevenness of the time delay for the power switch, and a large inrush current flows on account of the operational state of the inverter when turning the power switch ON or OFF and this can generate a negative effect; and when performing regular tests, maintenance, or inspections using remote monitoring, there are numerous problems in performing test, maintenance, or inspections that cannot be completed without turning off the power switch.
There are various technological advantages to the constantly sampled type error tracking power source technology given in the inventions of Japanese Patent Application First Publication No. H07-7950, Japanese Patent Application First Publication No. 2000-125575, and Japanese Patent Application First Publication No. 2000-341956; however, although the primary disclosure is the fundamental technology that relates to a single inverter, technology relating to inrush current prevention that easily occurs when operating a storage batter or condenser load, i.e. parallel operation of inverters, and methods for resolving the problematic issues relating thereto, are not yet disclosed.
The object of the present invention is to provide not only a constantly sampled type error tracking inverter technology, but an inverter that has the ability to make the output into a desired value by controlling the equivalent internal impedance by only the control parameters; and an inverter that can easily achieve the prevention of inrush current by applying these in the operation of parallel connected inverters; or an inverter that performs an interrupt or opening and equal work by substantially suspending (partially stopping of feeding, or a state like sleeping) or closing an arbitrary inverter by an extremely easy method without providing a negative influence due to a surge voltage or surge current when a plurality of inverters are in parallel operation.
Further, the measure to prevent cross-current as disclosed by the invention on Japanese Patent Application First Publication No. H08-223807, Japanese Patent Application First Publication No. H09-140148, Japanese Patent Application First Publication No. 2001-177997, Japanese Patent Application First Publication No. 2002-262577, Japanese Patent Application First Publication No. 2004-236496, and Japanese Patent Application First Publication No. H08-223808 has significant weaknesses in that various complicated functions or circuitry is required resulting in not only an increased size of the electric power source itself by increases the cost of the electric power source while reducing the reliability. Further, there is the problem that it is not possible to prevent the generation of cross-current due to the generation of noise during operation even when providing so many complicated functions and circuitry.
The object of the present invention is to provide not only constantly sampled type error tracking electric power source technology, but that which is suited to parallel operation of inverters that has the ability to control the equivalent internal impedance by only the control parameters and to devise a drastically simplified cross-current suppression technology in an electric power source consisting of a plurality of inverters connected in parallel that have the ability to control the equivalent internal impedance by only the control parameters.